The elements assembled around the central flexible core in the S-Z configuration may be of diverse types, as a function of the intended type of pipe: electrical cables, hydraulic pipes, thermal insulation tubes, steel tubes, optical fibers, etc.
The method of assembling such peripheral elements around the central flexible core of the pipe in an S-Z configuration is well known and entails periodically reversing the direction of applying the peripheral elements around the central core. Thus this assembly method combines helical winding in a first direction with winding in the opposite direction, as opposed to the standard helical assembly method in which the elements are applied helically around the central core in only one direction.
Applying the peripheral elements in an S-Z configuration produces the best compromise between mechanical strength resisting traction/compression forces, fatigue resistance, production cost and the overall size of the assembly device, compared to assembling peripheral elements wound helically. However, carrying out such assembly is relatively complex and requires vigilance with regard to the correct positioning of the peripheral elements assembled in this way. In fact, if the S-Z application of the peripheral elements is not done correctly, this can lead to mechanical damage inside the pipe structure. In particular, if the pipe is subjected to traction and/or compression forces when used in service, the peripheral elements may buckle and therefore lead to loss of the physical integrity of the pipe.
In this regard, the reversing point, which defines the position at which the direction of rotation of the peripheral elements is changed, constitutes a particularly critical and sensitive zone when assembling the peripheral elements.
Also, assembling the peripheral elements around the core in the S-Z configuration is immediately followed by the application of retaining adhesive tapes which immobilize the assembly in order to prevent any destranding of the peripheral elements on reversing direction. Now, the adhesive power of the retaining tapes may be affected by the temperature in the place where the pipe is produced, in particular at low temperatures of the order of a few degrees Celsius, or conversely in high temperature ranges, around 35° C., with the problematic consequence of not being able to retain in the proper position the peripheral elements applied in the S-Z configuration.
Given the constraints explained above linked to the S-Z assembly, which are likely to cause problems of physical integrity for the pipes concerned, it is therefore advisable to control the correct positioning of these elements during the production of the pipe and to be more specific just after the step of assembling the peripheral elements in the S-Z configuration.
Until now, in-production checking has been carried out manually by an operative. One example of the various known types of manual control during assembly of the structure including peripheral elements in an S-Z configuration is to have an operative control visually the value of the helix angle, i.e. the angle between the element being checked and the generatrix of the pipe. The operative uses for this a calibration instrument taking the form of a transparent material support on which is defined a tolerance range for the helix angle, corresponding to acceptable values of the angle relative to a horizontal axis marked on the support and intended to be superposed on the generatrix marked on the flexible core. Once the horizontal axis of the support has been superposed on the generatrix, the operative then verifies that the helix angle of the elements being checked lies within the tolerance range, the measurement using the calibration instrument being effected at the half-pitch of the helix. However, this control method is unsatisfactory in that the resulting diagnosis depends greatly on the visual acuity of the operatives and their ability to maintain a high level of concentration over time, as well as on the quality of the measuring instruments made available to them. Also, as a function of these conditions, errors may occur at the time of the various readings, cause production of the pipe to fail and, as a result, lead to the production of a new pipe. Moreover, it is also necessary to stop the production line regularly in order for the operative to be able to verify that the data read off is in fact consistent with the pipe currently being produced. This control method is therefore relatively unreliable and is both time-consuming and costly to set up.